A cross sectional diagram of a top-half portion of a prior art General Motors (GM) 4L80-E four speed transmission 100 is shown in FIG. 1. The 4L80-E transmission went into production in the early 1990s and the design of the transmission was based on GM's Turbo-Hydramatic (THM) 400 three speed transmission that was produced in the 1960s. The THM 400 was used in many full size passenger cars and trucks through the late 1980s and was well accepted because of its longevity and low maintenance. The 4L80-E incorporated some of the heavy-duty features of the THM 400 with an additional fourth gear with an over-driven ratio and also added a torque converter clutch, both additions providing improved fuel economy relative to the THM 400. The majority of the gears and clutches for reverse and first through third gears are shown within dashed line 102 in FIG. 1 and have a similar configuration to the THM 400. The additional components that were added to the THM design for providing a fourth gear are generally located to the left of the box formed by dashed line 102 in FIG. 1.
The gears and clutches for providing the additional over-driven fourth gear in the 4L80-E include an overdrive planetary gear set 104, which includes four planetary gears 106 (only one of which is visible in FIG. 1) and planetary carrier assembly 108. The fourth gear set also includes an overdrive roller clutch 110, an overrun clutch drum 112, and a sun gear 114 attached to the overrun clutch drum. The fourth gear set also includes a ring gear 116, which is attached to a forward clutch drum 118. The fourth gear set also includes a fourth clutch drum housing 120, fourth gear clutch 121, and overrun clutch 122. Engine torque in the GM 4L80 E is transmitted through the fourth gear set from torque converter turbine shaft 124 to the overdrive planetary gear set 104 via the geared coupling of the planetary carrier assembly 108 to the turbine shaft. In reverse and first through third gears, torque is transmitted from the planetary carrier assembly 108 to the overrun clutch drum 112 and sun gear 114 via either the overdrive roller clutch 110 or the overrun clutch 122. Torque is then transmitted from the sun gear 114 to the planetary gears 106 and ring gear 116 to the downstream gear sets within the box formed by dashed line 102. Thus, the entire load transmitted through transmission 100 at any given time when in gears 1-3 and reverse is borne by either overdrive roller clutch 110 or both the overdrive roller clutch and overrun clutch 122, depending on the mode of operation. In fourth gear, the fourth gear clutch 121 is engaged, causing the overrun clutch drum 112 to be stationary and torque is transmitted from the planetary carrier assembly 108 and planetary gears 106 directly to ring gear 116.
FIGS. 2 and 3 are partial hydraulic circuit diagrams for the prior art GM 4L80-E transmission 100 and compare the hydraulic configuration of the GM 4L80-E when in first gear when a manual valve 202 in two different positions 204. As shown in FIGS. 2 and 3, the GM 4L80-E manual valve 202 has seven positions 204, including Park (P), reverse (R), neutral (N), drive-4 (D) also referred to herein as the overdrive position or top gear position, manual-3 (3), manual-2 (2), and manual-1, the manual-3, 2, and 1 positions also referred to herein as lower gear positions. When the manual valve is in the overdrive or top gear position, during operation, transmission 100 will automatically shift between all forward gears, i.e., gears one through four, depending on the speed of the vehicle. When manual valve 202 is in manual-3 or 2, the transmission will only automatically shift up to the third or second gear, respectively, and when in manual-1, will remain in the first gear at all times. The position of the manual valve 202 is controlled by a gearshift in the vehicle (not illustrated). During normal vehicle operation, a user often places the manual valve in the drive-4 (overdrive) position and leaves it there when driving. A user may select manual 1-3 when, e.g., the vehicle is operating under a heavy load or when on a steep decline.
FIG. 2 shows the manual valve 202 in the overdrive or top gear position and shows the transmission 100 in first gear. As shown, pressurized transmission fluid is routed through a forward clutch feed passageway 206 to engage a forward clutch 208. The 4L80-E hydraulic system also includes a overrun clutch feed passageway 210 for routing pressurized transmission fluid to the overrun clutch 122 for engaging the overrun clutch in certain modes of operation. Manual valve 202 is in direct fluid communication with overrun clutch 122 via clutch feed passageway 210. As shown in FIG. 2, when the manual valve 202 is in the top gear position, the manual valve blocks pressurized transmission fluid from entering the overrun clutch feed passageway 210 and, therefore, the overrun clutch 122 is not engaged. Overrun clutch feed passageway 210 includes a check ball 212 (referred to by the OE manufacturer as the #1 check ball), which includes a seat passage 214 and orifice passage 216.
FIG. 3 shows the manual valve 202 in the lower gear manual-3 position and the transmission in first gear. As in FIG. 2, pressurized transmission fluid is routed through the forward clutch feed passageway 206 to the forward clutch 208. Unlike FIG. 2, with the manual valve 202 in the manual-3 position, the manual valve is no longer blocking pressurized transmission fluid from entering the overrun clutch feed passageway 210, and the pressurized transmission fluid engages the overrun clutch 122. As can be appreciated from a review of FIGS. 2 and 3, pressurized transmission fluid is also routed to the overrun clutch 122 in manual valve positions manual-2, and manual-1, in other words, in all of the lower gear positions, and is blocked when the manual valve is in park, reverse, neutral, and overdrive.
The prior art GM 4L80-E transmission 100, therefore, relies on the overdrive roller clutch 110 to provide a link between the planetary gear sets and output shaft 130 to propel the vehicle at all times when the manual valve 202 is in the reverse and overdrive positions. The overrun clutch 122 is not engaged to provide a backup for the overdrive roller clutch 110 unless the manual valve 202 is in the lower gear positions, i.e., manual-3, 2, or 1 positions. The overdrive roller clutch 110 is a non-hydraulic one-way clutch that includes rollers that slip or free-wheel when rotated in one direction and bind to prevent relative rotation in the other direction. The roller clutch 110, therefore provides improved fuel efficiency when coasting by not transmitting backward torque from the output shaft to the torque converter, avoiding engine braking. The overrun clutch 122 is a hydraulic multi-plate clutch pack such that when it is engaged, it does not allow relative rotation in either direction, thereby resulting in some engine braking when coasting and, therefore, may reduce fuel efficiency when engaged. The prior art GM 4L80-E overdrive roller clutch 110 is not as strong as the overrun clutch 122 and there has been a tendency for the overdrive roller clutch to fail.